Polyamide resins produced from a diamine and a dicarboxylic acid as a starting material are widely used in various fields. As a method for producing such a polyamide resin, common is a method in which first a nylon salt (polyamide salt) is prepared by using a diamine and a dicarboxylic acid, and the resulting nylon salt is polymerized under pressurized conditions.
However, when a nylon salt is polymerized under pressurized conditions to produce a polyamide resin, it takes a relatively long time for a practical molecular weight to be achieved. Consequently, a triamine, a by-product, is produced in the polyamide resin, to be a cause for gelation. Further, when the nylon salt is polymerized under pressurized conditions, the apparatus used for the polymerization is required to be of a pressurization type. Accordingly, disadvantageously the introduction of a pressurization-type apparatus, and the operation and maintenance of the apparatus increase the costs involved.
For the purpose of solving such a problem as described above, the polymerization of the nylon salt under normal pressure has been investigated. For example, JPH03-243623A describes a preparation of a polyamide resin by polymerizing under normal pressure a nylon salt obtained from equimolar amounts of 1,12-diaminododecane and a dicarboxylic acid.
However, the production method of JPH03-243623A is a production method restricted to the polymerization of the nylon salt prepared from a dicarboxylic acid and 1,12-diaminododecane, and is a technique feasible because of the boiling point of 1,12-diaminododecane as high as 304° C. On the contrary, when a nylon salt obtained from a dicarboxylic acid and a diamine having a boiling point of 300° C. or lower is polymerized by the technique described in JPH03-243623A, the diamine is volatilized from the reaction system, and the molar balance between the diamine and the dicarboxylic acid included in the nylon salt is lost. Consequently, the obtained polyamide resin does not attain the achievement of a high molecular weight, and is unsuitable for practical use.